DE-A-195 33 486 and DE-A-195 33 484 disclose catalyst supports or catalysts having a complex shape and processes for producing them, these catalyst supports or catalysts having a narrow monomodal or polymodal pore size distribution.
N-Vinylformamide is prepared by thermal dehydrocyanation of N-formylalanine nitril using a catalyst based on Al.sub.2 O.sub.3. The catalyst used hitherto was produced by impregnation of an Al.sub.2 O.sub.3 catalyst support with an active component and subsequent drying. Catalyst geometries which have been used hitherto are rings, wagon wheels and honeycombs. SiO.sub.2 was sometimes added to increase the mechanical strength of the Al.sub.2 O.sub.3 catalyst support. Thus, EP-A-0 206 193 (or DE-A 35 21 766) describes a honeycomb catalyst containing as main constituents from 30 to 95% by weight of an iron compound, calculated as Fe.sub.2 O.sub.3, and from 0.1 to 60% by weight of an aluminum, cerium and/or chromium compound, calculated as Al.sub.2 O.sub.3, CeO.sub.2, Cr.sub.2 O.sub.3, and/or from 1 to 50% by weight of an alkali metal compound, and also the use of this catalyst as a dehydrocyanation catalyst.
EP-A-0 208 929 concerns a catalyst fixed bed structure using the catalyst described in the above-cited applications for exothermic and endothermic chemical reactions having a high heat of reaction.
DE-A 34 43 463 relates to a process for preparing N-vinylformamide by pyrolysis of formylalanine nitril in the presence of solids as catalysts, these catalysts comprising alkali metal carbonates and alkaline earth metal carbonates, magnesium oxide, calcium oxide and barium oxide and particularly advantageous catalysts comprising alkali metal carbonates and/or alkaline earth metal carbonates on a-aluminum oxide as support.
During the catalytic synthesis of N-vinylformamide from N-formylalanine nitril, the active components such as alkali metal compounds and/or alkaline earth metal compounds gradually react with SiO.sub.2 at from 300.degree. C. to 600.degree. C. to give alkali metal and/or alkaline earth metal silicates which are inactive in respect of the reaction to be catalyzed and whose increasing formation reduces the mechanical strength of the catalyst. Accordingly, when the SiO.sub.2 -containing catalysts customarily employed are used, active component is continually lost during the reaction and, furthermore, the stability of the catalyst is reduced as the reaction progresses.
Extrusion enables SiO.sub.2 -free tubes or wagon wheel profiles to be produced. Previously, only SiO.sub.2 -containing shaped bodies could be produced by pressing, since the SiO.sub.2 -free pressed aluminum oxide bodies disintegrate after centering owing to their relatively low mechanical strength. Accordingly, without subsequent machining, only simple geometries of SiO.sub.2 -free Al.sub.2 O.sub.3 shaped bodies were obtainable in tube or rod form by means of extrusion. The use of simple catalyst geometries results, owing to unfavorable flow through the reactor bed, in high pressure drops and consequently unsatisfactory throughputs in catalytic reactions such as dehydrocyanation reactions.
In the case of the strongly exothermic reactions, it is also important for a high heat input into the catalyst bed to be ensured. In the case of reactions at subatmospheric pressure, the heat input is achieved predominantly by heat radiation and not by heat conduction. However, customary geometries of catalyst supports, eg. rings, wagon wheels and honeycombs, represent obstacles for heat transfer by radiation.